The present invention relates to a method for continuous production of elongated carbon bodies, and more particularly to carbon electrodes which are produced in direct connection to a smelting furnace wherein the electrodes are used, where a metallic casing containing unbaked carbonaceous electrode paste consisting of a particulate carbon material and a carbonaceous binder is being baked into a solid carbon body by lowering the metallic casing containing the carbonaceous electrode paste down through a baking furnace.
From Norwegian patent No. 154860 it is known a method for continuously production of elongated carbon bodies wherein a perforated metallic casing containing unbaked carbonaceous electrode paste consisting of a particulate solid carbon material and a carbonaceous binder, continuously or substantially continuously is lowered down through a baking furnace which is heated to a temperature between 500 and 1300xc2x0 C. The unbaked carbonaceous electrode paste is at this temperature baked into a solid carbon body. As the casing is being lowered down through the baking furnace new sections of casing are welded to the top of the metallic casing and further unbaked carbonaceous electrode paste is being filled into the casing.
The above described method can either be used for continuous production of elongated carbon bodies which after being baked in the baking furnace are cut into suitable lengths and which can be utilized as lining blocks for smelting furnaces, bottom blocks for cathodes in electrolytic cells for the production of aluminium and the like; or the method can be used for the production of continuous carbon electrodes in direct connection to a smelting furnace wherein the electrodes are being used. In the last mentioned case the baking furnace is arranged above the smelting furnace in such a way that the produced electrodes extend into the smelting furnace where they are consumed. The produced elongated carbon bodies may have any suitable cross-section. Carbon electrodes which are produced in direct connection with smelting furnaces usually have a circular cross-section.
By the known method new sections of casing are, as mentioned above, welded to the top of the casing. This is a labour intensive operation. Further, when the electrodes are produced in direct connection with a smelting furnace where the electrodes are consumed, the welding operation takes place in a hot and often polluted gas atmosphere. For electrodes where the casing is removed when the electrodes has been baked, the welding of the sections of the casing to each other means that the casing has to be cut horizontally in order to have it removed.
Upon heating the viscosity of the electrode paste is decreased whereby the electrode paste softens and upon further heating to a temperature between 500 and 1300xc2x0 C., the electrode paste is baked into a solid carbon body. During the heating the electrode paste shrinks and thus occupies a lesser volume. By the known methods where it is used sections of casing which are connected to each other by welding, the electrode paste will not have a sufficient downward flow to compensate for the shrinkage as the electrode paste will stick to the inner surface of the casing. There is thus a possibility that the baked carbon body will contain cavities, which will increase the possibility of electrode breakage when the carbon body is used as an electrode. Further, upon shrinkage of electrode paste which sticks to the inner surface of the casing, the casing may be locally deformed. This will be further enhanced as the casing during the further heating in the baking furnace will expand in axial direction whereby tensile stress can be introduced in the electrode paste.
From Swedish patent No. 112236 it is in connection with conventional self-baking electrodes for steel furnaces, known a method for the use of sections of casing which are filled with unbaked carbonaceous electrode paste. According to the Swedish patent each section of casing is filled with electrode paste prior to mounting the sections on the top of the electrode column. The lower part of each section of casing has a somewhat smaller diameter than the remaining part of the casing. When a new section of casing is mounted on the top of the electrode column, the lower part of the new section is installed in a telescopic way in the section of casing situated below the new section. In order to join the new section of casing to the electrode column, the area between the lower part of the new section of casing and the upper part of the electrode column is heated whereby the electrode paste in this area melts or softens whereby the electrode paste in the lower part of the new section of casing flows together with the electrode paste at the top of the electrode column. Thereafter the new section of casing is welded to the casing below. The baking of the electrode takes place in the area of electric power supply to the electrodes in the same way as for conventional self-baking electrodes.
The method disclosed in Swedish patent No. 112236 for joining sections of curings filled with electrode paste does, however, not solve the problems described above in connection with the electrode produced according to Norwegian patent No. 154860. Thus the use of the method for joining sections of casing according to the Swedish patent in connection with the method disclosed in Norwegian patent No. 154860, will not solve the problem of shrinkage of the electrode paste during the baking or the problem caused by expansion of the casing when the casing is heated in the baking furnace. Further, by using the method disclosed in the Swedish patent it will still be necessary to cut the casing horizontally in order to remove the casing below the baking furnace as it is a perequisite according to the Swedish patent that new sections of casing are welded to the casing below the new casing.
It is an object of the present invention to provide a method where it by the use of the method according to Norwegian patent No. 154860 is not necessary to weld new sections of casing to the casing below the new section of casing and where it is obtained an automatic compensation for shrinkage of the electrode paste and for extension of the casing during baking.
Accordingly, the present invention relates to a method for continuous production of elongated carbon bodies, particularly carbon electrodes which are produced in direct connection with the smelting furnace wherein the electrodes are consumed, where a metallic casing containing unbaked carbonaceous electrode paste comprising a particulate solid carbon material and a carbonaceous binder is continuously or substantially continuously lowered through a baking furnace which is heated to a temperature between 500 and 1200xc2x0 C., whereby the unbaked electrode paste is baked into a solid carbon body and where the casing is extended by joining new sections of casing on the top of the casing as the casing is lowered through the baking furnace, where the lower part of each section of casing has an outer diameter that is equal to or smaller than the inner diameter of the upper part of each section of casing, said method being characterized in that each new section of casing is mounted upon the section of casing below in such a way that the lower part of the new section of casing is positioned inside the casing of the upper part of the section of casing situated below the new section of casing and where the length of the lower part of each section of casing has such an extension that the new section of casing during baking freely can slide downwards in relation to the section of casing situated below the new section of casing a distance which at least compensate for the shrinkage of the electrode paste contained in the casing during baking of the carbon body in the baking furnace.
According to a preferred embodiment it is used sections of casing consisting of an upper cylindrical part and a lower cylindrical part where the outer diameter of the lower cylindrical part is equal to or smaller than the inner diameter of the upper cylindrical part.
It is preferred that the ratio between the length of the upper part and the lower part of the section of casing is between 1:1 to 1000:1 and more particularly between 3:1 and 12:1.
According to another embodiment it is used sections of casing where at least the lower part of each section of casing has a conical shape such that the outer diameter of the conical part of the section of casing is smaller than the diameter of the top of the casing.
By the method of the present invention each section of casing is thus not rigidly affixed to the section of casing below, but is only loosily placed on the top of the section of casing situated below. As the electrode paste in the section below the upper section shrinks during the baking, the weight of the upper section of casing will cause the section of casing to freely slide down into the casing in the section of casing situated below. Shrinkage of the electrode paste will thus not cause local deformation of the casing.
By the method of the present invention the removal of sections of casing after baking of the electrode in the baking furnace is substantially simplified as one only has to cut the casing vertically.
According to another embodiment of the present invention each section of casing is filled with unbaked electrode paste to such a level that the distance from the level of electrode paste to the top of each section of casing is less than the length of the lower part of the sections of casing. The sections of casing can be filled with unbaked electrode paste prior to or after the sections are installed. In this way the lower part of the sections of casing will, when mounted, rest upon the unbaked electrode paste in the section of casing situated below.